The present invention relates to electro-optical liquid-crystalline displays, in particular STN displays having low addressing voltages.
In liquid-crystal displays of this type, the liquid crystals are used as dielectrics whose optical properties change reversibly on application of an electric voltage. Electro-optical displays which use liquid crystals as media are known to the person skilled in the art. These liquid-crystal displays use various electro-optical effects. The most common of these are the TN effect (twisted nematic, having a homogeneous, virtually planar initial alignment of the liquid crystals and a nematic structure which is twisted by about 90xc2x0) and the STN effect (supertwisted nematic) and the SBE effect (supertwisted birefringence effect). In these and similar electro-optical effects, liquid-crystalline media of positive dielectric anisotropy (xcex94∈) are used.
In STN displays, which, in the present application, include all common and known types of display with relatively high twist, such as, for example, SBE (supertwisted birefringent effect), GH (guest/host), STN and OMI (optical mode interference) displays, as well as compensated STN displays, such as DSTN (double layer STN) and film-compensated STN displays, the liquid-crystal director is twisted from one side of the liquid-crystal layer to the other by a given angle of from greater than 90xc2x0, typically of 180xc2x0 or more, up to 600xc2x0, typically up to 270xc2x0. This is achieved on the one hand by corresponding alignment of the preferential directions of the liquid-crystal alignment of the two substrates to one another. The preferential direction of the alignment on the substrates is achieved by an anisotropic pretreatment, typically by rubbing a special, usually polymeric organic layer in one direction, or by vapour-deposition of SiOx at an angle. On the other hand, a chiral liquid-crystal medium is employed which consists of mesogenic chiral substances or, most widespread, consists of a non-chiral medium to which a chiral substance (a so-called dopant) is added. The latter alternative is usually preferred since, through variation of the concentration of the dopant, it enables the twist of the liquid-crystal layer to be set to virtually any desired values. It must be ensured here that the ratio of the layer thickness of the liquid-crystal layer (d) to the cholesteric pitch of the liquid-crystal (P) is sufficiently great in order to produce the desired twist. To this end, a twist value of more than 90xc2x0 (or d/P=90xc2x0/360xc2x0=0.25) below the desired twist is generally set. This so-called geometrical limit is thus, for example, 0.5xe2x88x920.25=0.25 for cells having a twist of 180xc2x0 and 0.667xe2x88x920.25=0.417 for a cell having a twist of 240xc2x0. The upper geometrical limit is in each case at a twist which is 180xc2x0 higher, i.e. at a d/P value which is 0.5 higher. On application of an electric voltage, however, the cholesteric pitch increases, and thus the lower limit of the d/P ratio increases. Although the same effect can also occur at the upper limit, this is virtually impossible to utilize since an undesired electro-optical effect in the form of a refractive-index grid, the so-called striped transition, perpendicular to the director orientation in the centre of a liquid-crystal layer occurs on application of an electric voltage in the region of relatively high doping. This effect significantly reduces the upper limit of possible doping and is usually, in particular in the case of relatively high twist angles, much more pronounced than the increase in the lower limit on application of the voltage.
For fault-free operation of STN displays, however, a uniform transition of the liquid-crystal layer from the initial alignment to the final alignment, if possible over the entire display area, but at least over a pixel, is required. During this transition, the director of the liquid-crystal layer within each imaginary parallel sub-layer, irrespective of the location, moves toward one another in the same direction and at the same angle. This transition is also known as the Freedericks transition. However, this desired transition does not occur for all possible parameter combinations. Depending both on the properties of the liquid crystal and on the design of the display, an undesired transition known, owing to its optical appearance, as striped domain transition/distortion, occurs on application of an electric voltage. This transition takes precedence over the desired Freedericks transition if the liquid-crystal parameters, in particular the elastic constants and the dielectric anisotropy, are favorable for a steep electro-optical characteristic line. It is furthermore favored by a large d/P ratio and depends not least on the twist angle used and the surface tilt angle. The larger the twist angle, the greater the surface tilt angle has to be in order to enable stable operation of the display. Tilt angles of at least 2xc2x0, 3xc2x0 or 4 to 5xc2x0 are typically used at the twist angles of 180xc2x0, 220xc2x0 and 240xc2x0 that are generally frequently used.
Since the operating voltage in displays, i.e. also in displays with these effects, should generally be as low as possible, use is made of liquid-crystal media of high dielectric anisotropy, which generally consist predominantly and usually even very substantially of liquid-crystal compounds having the corresponding dielectric anisotropy, i.e. compounds of positive dielectric anisotropy in the case of dielectrically positive media. If need be, significant amounts of dielectrically neutral liquid-crystal compounds are typically employed. Liquid-crystal compounds with the sign of the dielectric anisotropy opposite to the dielectric anisotropy of the medium are generally employed extremely sparingly or not at all.
An exception is formed here by STN displays, which are also the subject-matter of the present application. In STN displays, dielectrically positive liquid-crystal media which comprise dielectrically negative liquid-crystal compounds can be employed, for example in accordance with DE 41 00 287, in order to increase the steepness of the electro-optical characteristic line.
The pixels of the liquid-crystal displays can be addressed directly, time-sequentially, i.e. in time multiplex mode, or by means of a matrix of active, electrically nonlinear elements.
In STN displays, addressing in time multiplex mode is the most wide-spread. In this mode, the columns and rows of a matrix-form arrangement of liquid-crystal switching elements are addressed by means of an Alt and Pleschko addressing scheme. In this case, the liquid-crystal medium of the liquid-crystal display elements reacts to the root mean square (rms) of the addressing voltage. Particularly at relatively high multiplex ratios and in the case of very fast-switching liquid-crystal switching elements, however, this no longer applies. The addressing here can alternatively be carried out by xe2x80x9cmultiple line addressingxe2x80x9d, by means of xe2x80x9cactive addressingxe2x80x9d or the so-called xe2x80x9cimproved Alt-Pleshko addressingxe2x80x9d.
The term xe2x80x9clow multiplex drivexe2x80x9d is commonly used for multiplex ratios of 1:32 or less, the term xe2x80x9cmid multiplex drivexe2x80x9d is commonly used for multiplex ratios in the range from about 1:48 to 1:100, and the term xe2x80x9chigh multiplex drivexe2x80x9d is commonly used for multiplex ratios of about 1:128 or more (for example 1:240, 1:400 or 1:480).
The steepness of the electro-optical characteristic line of the liquid-crystal cell must be sufficiently large (the numerical value V90/V10 must be correspondingly small) in order to be able to address the requisite number of rows. This also applies in the case of liquid-crystal displays having low addressing voltages. In these, however, the possible variations of the liquid-crystal medium are subject to relatively narrow limits. On the one hand, a large proportion of highly dielectrically positive compounds is required in order to achieve the requisite low threshold voltages. This results in a large proportion of the constituents of the medium being prespecified by substances which do not result in the best steepness of the characteristic line and at the same time restrict the latitude for use of compounds having corresponding elastic constants and makes the use of dielectrically negative compounds virtually impossible. In view of the fact that the liquid-crystal displays must have an adequate working temperature range and short response times, the choice of possible compounds is greatly restricted even for the remaining constituents of the liquid-crystal media.
It can thus be seen that there is a demand for liquid-crystal displays having an adequate working temperature range and short response times, in particular at low operating voltages and sufficiently great steepness of the electro-optical characteristic line.
Surprisingly, it has now been found that this can be achieved by the use of a corresponding liquid-crystal medium having an optimized alignment layer.
The most important parameters of the alignment layer are its dielectric constant, its surface energy and its layer thickness. In addition, its refractive index, its transparency and its color also have to be taken into account.
The alignment layers used are generally, and also in accordance with the present invention, rubbed organic layers, in particular polyimides. For STN displays, which are the subject-matter of the present invention, use is frequently preferably made of polyamidic acids, which are not imidated until they are on the substrate.
Preference is given to organic alignment layers and particularly preferably polyimides. In accordance with the present application, preference is given to the thinnest possible alignment layers. Alignment layers (ALs) having a layer thickness (d(AL)) in the range from 5 nm to 100 nm, particularly in the range from 7 nm to 80 nm, very particularly preferably in the range from 8 nm to 60 nm, and especially preferably in the range from 10nm to 25 nm, are typically used.
The upper limit for the layer thickness of the alignment layer is determined, in particular, by the desired steepness. The lower the layer thickness of a given alignment layer for a given liquid-crystal medium, the smaller is also the steepness of the electro-optical characteristic line.
The lower limit of the layer thickness of the alignment layer is determined by the applicational properties. Thus, generation of the requisite preferential direction by rubbing is no longer possible in the case of very thin layers. Various defects may occur individually or together. The most typical defects are inadequate or nonuniform alignment or the occurrence of small holes, known as pinholes, in the alignment layer.
Alignment layers which can be used in accordance with the present invention are the usual commercial polyimides. Examples which may be mentioned here are SE 150, SE 3140 and SE 4110 from Nissan Chemicals, Japan, SN-7100, SN-7200 and SN-7300 from HDM (Hitachi-DuPont Microsystems), Japan, and polyimides from Japan Synthetic Rubber, Japan.
A significant effect on the results is exerted by the dielectric constant of the alignment layer used. The dielectric constant of the polyimide SE 3140 is, for example, 3.0 and that of SE 150 is 2.9.
Suitable for use in displays according to the invention are, in particular, liquid-crystal media of high dielectric anisotropy and with a relatively broad operating- and storage-temperature range. Thus, the shelf life at xe2x88x9230xc2x0 C. in liquid-crystal cells having a liquid-crystal layer thickness which is suitable for the requisite optical retardation is preferably greater than or equal to 500 hours and particularly preferably greater than or equal to 1000 hours. Very particular preference is given to liquid-crystal media whose shelf life at xe2x88x9240xc2x0 C. is 1000 hours or more.
Preference is given to liquid-crystal media which comprise one or more compound(s) of the formula I 
in which
R1 is alkyl or alkoxy having 1 to 7 carbon atoms or alkoxyalkyl, alkenyl or alkenyloxy having 2 to 7 carbon atoms, preferably alkyl or alkoxyalkyl, and
Y1 is H or F, preferably H.
In a preferred embodiment, the liquid-crystal media employed in accordance with the present invention comprise
a) one or more highly dielectrically positive compound(s) of the formula (I) 
xe2x80x83in which
R1 is alkyl or alkoxy having 1 to 7 carbon atoms, preferably n-alkyl or n-alkoxy, particularly preferably having 2 to 5 carbon atoms, or alkoxyalkyl, alkenyl or alkenyloxy having 2 to 7 carbon atoms, preferably 1E-alkenyl, 1-alkenyloxy or straight-chain alkoxyalkyl, preferably having 2 to 5 carbon atoms, and particularly preferably alkyl or alkoxyalkyl, and
Y1 is H or F, preferably H,
xe2x80x83and/or
b) one or more dielectrically positive compound(s) of the formula II 
xe2x80x83in which
R2, Y21 and Y22 are as defined above under the formula I for R1 and Y1 respectively, and Y21 and Y22 are preferably both H or both F,
xe2x80x83and/or, preferably and
c) one or more dielectrically neutral compound(s) of the formula III 
xe2x80x83in which
R31 and R32 are each, independently of one another, as defined above under the formula I for R1, and
Z31, Z32 and Z33 are each, independently of one another, xe2x80x94CH2CH2xe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94COOxe2x80x94 or a single bond, 
o and p, independently of one another, are 0 or 1,
xe2x80x83but preferably
R31 and R32 are each, independently of one another, alkyl or alkoxy having 1-5 carbon atoms or alkenyl having 2-5 carbon atoms, 
xe2x80x83and very particularly preferably at least two or these rings are 
xe2x80x83where very particularly preferably two adjacent rings are linked directly, 
Preferably, the media contain one or more compounds selected from the group of compounds a) (Formula I) and b) (Formula II) and optionally one or more of compounds c) (Formula III). Further, the media preferably contain one or more compounds selected from a) or b) and one or more compounds of c), most preferably they contain one or more compounds of each of a), b) and c).
In a further preferred embodiment, the liquid-crystal medium employed additionally comprises one or more compound(s) of the formula IV 
in which
R41 and R42 are each, independently of one another, alkyl or alkoxy having 1 to 7 carbon atoms, preferably n-alkyl or n-alkoxy, preferably having 1 to 5 carbon atoms, or alkoxyalkyl, alkenyl, alkenyloxy or alkynyl having 2 to 7 carbon atoms, preferably straight-chain alkoxyalkyl, 1E-alkenyl or 1E-alkenyloxy, preferably having 1 to 5 carbon atoms, 
Z4 is COO, CH2CH2, xe2x80x94Cxe2x89xa1Cxe2x80x94 or preferably a single bond, where the phenyl rings, independently of one another, may optionally be monosubstituted or disubstituted by F, and
n4 is 0, 1 or 2, preferably 0 or 1.
The liquid-crystal medium employed optionally comprises one or more compound(s) of the formula V 
in which
R5 is alkyl having 1 to 9 carbon atoms, preferably n-alkyl, preferably having 2 to 7 carbon atoms,
Z5 is xe2x80x94CH2CH2xe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94 or xe2x80x94COOxe2x80x94, preferably xe2x80x94CHxe2x95x90CHxe2x80x94 or xe2x80x94COOxe2x80x94, particularly preferably xe2x80x94COOxe2x80x94, and
Y51 and Y52 are each, independently of one another, H or F, preferably one is H and the other is F or both are F, particularly preferably both are H.
The liquid-crystal medium employed preferably comprises one or more compound(s) of the formula VI 
in which
R6 is as defined above under the formula I for R1, 
Z61 and Z62 are each, independently of one another, xe2x80x94CH2xe2x80x94CH2xe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94COOxe2x80x94 or a single bond,
X6 is F, Cl, OCF2H, OCF3 or CF3,
Y61 and Y62 are each, independently of one another, H or F, and
n6 is 0, 1 or 2, preferably 0 or 1.
Particular preference is given to liquid-crystal media which comprise one or more compound(s) of the formula I selected from the group consisting of the compounds of the subformulae I1 to I3 
in which
n is from 1 to 9, preferably from 2 to 7, and
m is from 1 to 3
The liquid-crystal media employed in accordance with the invention preferably comprise one or more compound(s) of the formula II selected from the group consisting of the compounds of the subformulae II1 to II3: 
in which
R2 is as defined above under the formula II.
The liquid-crystal medium employed particularly preferably comprises one or more compound(s) selected from the group consisting of the compounds of the formulae III1 to III3: 
in which R31, R32, Z31, Z32, 
are each as defined above under the formula III.
The liquid-crystal medium employed especially preferably comprises one or more compound(s) selected from the group consisting of the compounds of the formulae III1a to III1d, III2a to III2e, III3a to III3d and III4a: 
in which n and m are each, independently of one another, from 1 to 5, and o and p are each, independently thereof and from one another, are from 0 to 3, 
in which R31 and R33 are each as defined above under the formula III1, and the phenyl rings may optionally be fluorinated, but not in such a way that the compounds are identical with those of the formula II and their subformulae. R31 is preferably n-alkyl having 1 to 5 carbon atoms, particularly preferably having 1 to 3 carbon atoms, and R32 is preferably n-alkyl or n-alkoxy having 1 to 5 carbon atoms or alkenyl having 2 to 5 carbon atoms. Of these, particular preference is given to the compounds of the formulae III1a to III1d.
The liquid-crystal medium employed preferably comprises one or more compound(s) selected from the group consisting of the compounds of the formulae IV1a to IV1c, IV2a to IV2d and IV3a to IV3b: 
in which
R41 and R42 are as defined above under the formula IV.
The liquid-crystal medium employed preferably comprises one or more compound(s) selected from the group consisting of the compounds of the formulae V1a to V1c and V2a to V2c: 
in which
R5is as defined above under the formula V.
The liquid-crystal medium employed preferably comprises one or more compound(s) selected from the group consisting of the compounds of the formulae VI1a to VI1d, VI2a to VI2d and VI3a and VI3b: 
in which
R6 is as defined above under the formula VI.
The liquid-crystal media employed in accordance with the invention preferably comprise from 40% to 75% of one or more, preferably more, preferably highly dielectrically positive compound(s) selected from the group consisting of the compounds of the formulae I and II, preferably from the group consisting of the compounds of the formulae I1 and II2.
In a preferred embodiment, the liquid-crystal media employed in accordance with the invention comprise in total, based on the mixture as a whole,
from 40% to 70% of compounds of the formula I
from 0% to 30% of compounds of the formula II, and
from 0% to 40% of compounds of the formula III.
Here, as in the entire present application, the term xe2x80x9ccompoundsxe2x80x9d, for clarification also written as compound(s), is taken to mean, unless expressly stated otherwise, both one compound and a plurality of compounds.
The individual compounds of Formulas I to VI here are generally employed in concentrations of from 1% to 30%, preferably from 2% to 20% and particularly preferably from 4% to 16%.
In a particularly preferred embodiment, the compounds of the various formulae are employed in the liquid-crystal media employed in accordance with the invention as summarized in the following table.
The liquid-crystal media in this embodiment very particular preferably comprise in total
from 50% to 58% of compounds of the formula I,
from 7% to 16% of compounds of the formula II,
from 2% to 20% of compounds of the formula III,
from 7% to 14% of compounds of the formula IV,
from 0% to10% of compounds of the formula V, and
from 10% to 25% of compounds of the formula VI.
In a particularly preferred embodiment, which may be identical and preferably is identical with the preferred embodiments described above for the preferred concentrations ranges, the liquid-crystal media comprise
one or more compounds of the formula I1 and/or
one or more compounds of the formula II3 and/or
one or more compounds selected from the group consisting of the compounds of the formula III1a to III1c and/or
one or more compounds selected from the group consisting of the compounds of the formulae III2 to III3 and/or
one or more compounds selected from the group consisting of the compounds of the formulae VI1d, VI2b, VI2c, VI3a and VI3b.
Particular preference is given here to liquid-crystal media which satisfy one or more of the following conditions. The media comprise
one or more compounds of the formula I1, in particular in each case in concentrations of from 3% to 15% per compound,
one or more compounds of the formula II3, in particular in each case in concentrations of from 5% to 20% per compound,
one or more compounds of the formula III1, in particular in each case in concentrations of from 3% to 15% per compound,
one or more compounds of the formula III2, in particular in each case in concentrations of from 3% to 12% per compound, preferably in each case at least one compound in which R31 is alkenyl,
one or more compounds of the formulae III1a and/or III1c, in particular in concentrations of from 4% to 15% per compound,
one or more compounds of the formula III2a,
one or more compound(s) of the formula VI2b and/or of the formula VI3a.
The above-mentioned preferred concentration ranges particularly preferably also apply to this preferred combination of compounds.
The liquid-crystal media used in accordance with the invention preferably have nematic phases of in each case at least from xe2x88x9220xc2x0 C. to 70xc2x0 C., preferably from xe2x88x9230xc2x0 C. to 80xc2x0 C. and very particularly preferably from xe2x88x9240xc2x0 C. to 80xc2x0 C. The term xe2x80x9chave a nematic phasexe2x80x9d here means firstly that no smectic phase and no crystallization are observed at low temperatures at the corresponding temperature and secondly that no clearing occurs on heating from the nematic phase. The investigation at low temperatures is carried out in a flow viscometer at the corresponding temperature and checked by storage in test cells having a layer thickness corresponding to the electro-optical application for at least 100 hours. At high temperatures, the clearing point is measured by conventional methods in capillaries.
Furthermore, the liquid-crystal media used in accordance with the invention are characterized by optical anisotropy values which are particularly suitable for STN displays having conventional layer thicknesses. The birefringence values are in the range from 0.100 to 0.180, preferably in the range from 0.120 to 0.170 and very particularly preferably in the range from 0.120 to 0.160 and most preferably in the range from 0.130 to 0.150.
In addition, the liquid-crystal displays according to the invention have relatively low threshold voltage values of less than or equal to 1.8 V, preferably less than or equal to 1.7 V, particularly preferably less than or equal to 1.5 V and very particularly preferably less than or equal to 1.0 V.
The steepness values of the electro-optical-characteristic line of the liquid-crystal displays are preferably less than or equal to 1.100, particularly preferably less than or equal to 1.080, very particularly preferably less than or equal to 1.060 and in particular less than or equal to 1.040. They are generally at values of 1.020 or greater.
The use of alignment layers of low layer thickness improves the steepness of the electro-optical characteristic line of the displays. The difference in the steepness of the characteristic line from 1 (V90/V10xe2x88x921) of the displays according to the invention is preferably less than 80% of the corresponding value of an otherwise identical display having alignment layers with a layer thickness of 100 nm. This value is particularly preferably less than 70% and very particularly preferably at most 50% of the comparative value of the comparative display having an alignment layer with a thickness of 100 nm. Particular preference is given to liquid-crystal displays in which the difference of the steepness values from 1 is 90% or less, preferably 75% or less and particularly preferably 50% or less than the corresponding value of a comparative display having a layer thickness of the alignment layer of 70 nm.
The alignment layer preferably has a refractive index in the range from 1.550 to 1.800, particularly preferably in the range from 1.590 to 1.750 and very particularly preferably in the range from 1.600 to 1.700.
The transmission by the alignment layer (where twice the layer thickness is taken into account since both substrates of the display carry an alignment layer) at a wavelength of 390 nm is preferably 90% or more, particularly preferably 95% or more, very particularly preferably 97% or more and in particular 99% or more.
The various preferred ranges for the values of the individual physical properties of the alignment layer and of the liquid-crystal material are preferably also observed both with one another and combined amongst one another. Thus, for example, the liquid-crystal media according to the invention have, in particular, the following property combinations:
The twist angle of the liquid-crystal layer of the liquid-crystal displays according to the invention is preferably from 180xc2x0 C. to 290xc2x0 C., particularly preferably from 180xc2x0 to 270xc2x0, very particular preferably from 180xc2x0 to 260xc2x0 and especially preferably from 200xc2x0 to 245xc2x0.
The surface tilt angle is preferably in the range from 20xc2x0 to 25xc2x0, particularly preferably in the range from 3xc2x0 to 15xc2x0, very particularly preferably in the range from 3xc2x0 to 9xc2x0 and in particular in the range from 4xc2x0 to 7xc2x0.
The layer thickness of the liquid-crystal layer of the displays according to the invention is preferably from 3xcexcm to 8 xcexcm, particular preferably from 4 xcexcm to 6.5 xcexcm.
The term xe2x80x9calkylxe2x80x9d preferably covers straight-chain and branched alkyl groups having 1-7 carbon atoms, in particular the straight-chain groups methyl, ethyl, propyl, butyl, pentyl, hexyl and heptyl. Groups having 2-5 carbon atoms are generally preferred.
The term xe2x80x9calkenylxe2x80x9d preferably covers straight-chain and branched alkenyl groups having 2-7 carbon atoms, in particular the straight-chain groups. Particularly preferred alkenyl groups are C2-C7-1E-alkenyl, C4-C7-3E-alkenyl, C5-C7-4-alkenyl, C6-C7-5-alkenyl and C7-6-alkenyl, in particular C2-C7-1E-alkenyl, C4-C7-3E-alkenyl and C5-C7-4-alkenyl. Examples of further preferred alkenyl groups are vinyl, 1E-propenyl, 1E-butenyl, 1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 3-butenyl, 3E-pentenyl, 3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl, 4E-hexenyl, 4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Groups having up to 5 carbon atoms are generally preferred.
The term xe2x80x9coxaalkylxe2x80x9d or alkoxyalkyl preferably covers straight-chain radicals of the formula CnH2n+1xe2x80x94Oxe2x80x94(CH2)m, in which n and m are each, independently of one another, from 1 to 6, and the sum of m+n is up to 7. Preferably, n=1 and m is from 1 to 6.
Through a suitable choice of the meanings of the parameters of the compounds, in particular of R1, Y1, R2, Y21, Y22 but also the parameters of the other compounds, the addressing times, the threshold voltage, the steepness of the transmission characteristic lines, etc., can be modified in the desired manner. For example, 1E-alkenyl radicals, 3E-alkenyl radicals, 2E-alkenyloxy radicals and the like generally result in shorter addressing times, improved nematic tendencies and a higher ratio of the elastic constants k33 (bend) and k11 (splay) compared with alkyl or alkoxy radicals. 4-alkenyl radicals, 3-alkenyl radicals and the like generally give lower threshold voltages and smaller values of k33/k11 compared with alkyl and alkoxy radicals.
A xe2x80x94CH2CH2xe2x80x94 group generally results in higher values of k33/k11 compared with the single covalent bond. Higher values of k33/k11 facilitate, for example, flatter transmission characteristic lines in TN cells with a 90xc2x0 twist (for achieving grey shades) and steeper transmission characteristic lines in STN, SBE and OMI cells (greater multiplexability), and vice versa.
In the present application, the term xe2x80x9cdielectrically very highly positive compoundsxe2x80x9d denotes compounds having a xcex94∈ of  greater than 20, the term xe2x80x9cdielectrically highly positive compoundsxe2x80x9d denotes compounds having a xcex94∈ in the range from less than 20 to xcex94∈=10, the term xe2x80x9cdielectrically slightly positive compoundsxe2x80x9d denotes compounds having a xcex94∈ in the range from less than 10 to xcex94∈ greater than 1.5, the term xe2x80x9cdielectrically neutral compoundsxe2x80x9d denotes those in which xe2x88x921.5xe2x89xa6xcex94∈xe2x89xa61.5, and the term xe2x80x9cdielectrically negative compoundsxe2x80x9d denotes those in which xcex94∈ is  less than xe2x88x921.5. The dielectric anisotropy of the compounds is determined here by dissolving 10% of the compounds in a liquid-crystalline host and determining the capacitance of this mixture in at least one test cell in each case with a thickness of 10 xcexcm with homeotropic and with homogeneous surface alignment at 1 kHz. The measurement voltage is typically from 0.2 V to 1.0 V, but is always less than the capacitive threshold of the respective liquid-crystal mixture.
The host mixture used for dielectrically positive compounds is ZLI-4792, and the host mixture used for dielectrically neutral and dielectrically negative compounds is ZLI-3086, both from Merck KGaA, Germany. The values for the respective compounds to be investigated are obtained from the change in the dielectric constants of the host mixture after addition of the compound to be investigated and extrapolation to 100% of the compound employed.
The term xe2x80x9cthreshold voltagexe2x80x9d usually relates to the optical threshold for 10% relative contrast (V10).
All concentrations in this application, unless explicitly stated otherwise, are given in per cent by weight and relate to the corresponding mixture as a whole. All physical properties are and were determined in accordance with xe2x80x9cMerck Liquid Crystals, Physical Properties of Liquid Crystalsxe2x80x9d, as of Nov. 1997, Merck KGaA, Germany, and apply for a temperature of 20xc2x0 C., unless explicitly stated otherwise. xcex94n is determined at 589 nm and xcex94∈ at 1 kHz.
The threshold voltages and the other electro-optical properties were determined in test cells manufactured at Merck KGaA, Germany, using white light and using a commercial Autronic-Melchers DMS-301 measuring instrument. To this end, cells were selected, depending on xcex94n of the liquid crystals, having a thickness corresponding to an optical retardation dxc2x7xcex94n of the cells of about 0.85 xcexcm. The cells had a twist angle of 240xc2x0. The d/P value was set at 0.53 by means of the chiral dopant S-811 (Merck KGaA, Darmstadt, Germany). The cells were operated in so-called xe2x80x9cyellow modexe2x80x9d. For the standard characterization of the electro-optical properties of the liquid-crystal mixtures, in particular of the characteristic voltages, such as the threshold voltage, cells containing SE 4110 from Nissan Chemicals, Japan, with a layer thickness of the polyimide of about 30 nm as alignment layer were used. The characteristic voltages were all determined with perpendicular observation. The threshold voltage was indicated as V10 for 10% relative contrast, the mid-grey voltage Vxe2x95x90, was indicated for 50% relative contrast and the saturation voltage V90 was indicated for 90% relative contrast.
In the case of some liquid-crystal media, the threshold voltage was additionally determined as the capacitive threshold V0 (also known as the Freedericks threshold).
The liquid-crystal media according to the invention may, if desired, also comprise furthermore additives in the conventional amounts. The amount of these additives employed, like that of the chiral dopants, is in total from 0% to 10%, based on the amount of the mixture as a whole, preferably from 0.1% to 6%. The concentration of the individual compounds employed is preferably from 0.1 to 3%. The concentration of these and similar additives is not taken into account when specifying the concentrations and the concentration ranges of the liquid-crystal compounds in the liquid-crystal media.
The liquid-crystal media employed in accordance with the invention consist of a plurality of compounds, preferably from 3 to 30, particularly preferably from 6 to 20 and very particularly preferably from 10 to 16 compounds, which are mixed in a conventional manner. In general, the desired amount of the components used in lesser amount are dissolved in the components making up the principal constituent, advantageously at elevated temperature. If the selected temperature is above the clearing point of the principal constituent, the completeness of the dissolution process is particularly easy to observe. However, it is also possible to prepare the liquid-crystal mixtures by other conventional methods, for example using premixtures or from a so-called xe2x80x9cmultibottle systemxe2x80x9d. By means of suitable additives, the liquid-crystal phases employed in accordance with the invention can be modified in such a way that they can be employed in any type of STN display that has been disclosed hitherto.
The entire disclosure of all applications, patents and publications, cited above and below, and of corresponding German Application No. 100 188 882.6, filed Apr. 14, 2000 is hereby incorporated by reference.
The examples below serve to illustrate the invention without representing a limitation. In the examples, the melting point T (C,N), the transition from the smectic (S) to the nematic (N) phase T(S,N) and the clearing point T (N,I) of a liquid-crystal substance are given in degrees Celsius. The percentages are, unless explicitly stated otherwise, per cent by weight above and below, and the physical properties are the values at 20xc2x0 C., unless explicitly stated otherwise.
All temperature values indicated in this application arexc2x0 C. and all temperature differences are correspondingly differential degrees, unless explicitly stated otherwise.
In the present application and in the following examples, the structures of the liquid-crystal compounds are indicated by means of acronyms, the transformation into chemical formulae taking place in accordance with Tables A and B below. All radicals CnH2n+1 and CmH2m+1 are straight-chain alkyl radicals having n and m carbon atoms respectively. The coding in Table B is self-evident. In Table A, only the acronym for the parent structure is given. In individual cases, the acronym for the parent structure is followed, separated by a dash, by a code for the substituents R1, R2, L1 and L2: